-Stage 1.2.3-

Sedimentary Rocks

Definition - Sedimentary Rocks form from the cementation of fragments of other rocks including igneous, metamorphic and other sedimentary rocks. They also form from the cementation  and precipitation of animal byproducts.

Sedimentary Rocks Page Layout

• Websites

• Types of Sedimentary Rocks

• Identifying Sedimentary Rocks

• Petrogenesis (Creating Rocks)

• Clastic Sedimentary Rocks Descriptions

• Chemical Sedimentary Rocks Descriptions

• Maturity

Websites

Types of Sedimentary Rocks

• There are 2 types of Sedimentary Rocks

  • Clastic - Rocks that are formed from the fragments (Clasts) from any other rocks.

   

  • Chemical - There are two types of Chemical Sedimentary Rocks

    • Biogenic - Rocks that are formed from the activity of life forms (biological).

     

    • Precipitate - Rocks that are formed from naturally precipitated minerals.

Identifying Sedimentary Rocks

The basic types of Sedimentary Rocks are listed below

Petrogenesis (Creating Rocks)

- Sedimentary Rocks Formation Locations -

Clastic Sedimentary Rocks

Most are formed on or near dry land due to the processes of running water. Starting with the top of the mountain:

• Breccia - These rocks often form near mountains peaks where landslides occur. The landslides pile up large piles of debris that contain large and small angular fragments of rock. The large angular fragments are sometime cemented into a rock. Other times the debris is carried downstream starting with the braided river.

Picture from seis.natsci.csulb.edu

• Conglomerate - These rocks form along braided rivers where the debris from the mountain peaks are rounded in the streams and where sorting starts to take place. Usually the large pebbles are left behind while smaller debris is carried further downstream.

• Sandstone - There are multiple varieties of sandstone that all depend on the type of debris in the area and how long the materials have been transported.

  • Arkose Sandstone - Arkose is defined as having at least 25% Potassium Feldspar (K-Spar) causing the rock to be generally pink or red. They form in close relation to granites which have high amounts of K-Spar and are often associated with breccias.

   

  

  Picture from gpc.edu

   

  • Lithic Sandstone - Formed a little downstream from the Arkose causing the content to have a higher ratio of rock (lithic) fragments. K-Spar dissolves easily hence it not lasting far from the source rock.

   

  

  Picture from csmres.jmu.edu

   

  • Quartz Arenite (Quartz Sandstone) - Out of all the sandstones this one has been transported the farthest. Usually consisting of >90% quartz since that is the most abundant mineral on earth and it is harder than all the other common minerals. Typically this rock forms in beaches or deserts where quartz sand is abundant. The farther offshore you go the smaller the sand in the sandstone.

Picture from csmres.jmu.edu

• Siltstone - Forms in a couple of places but is usually rare. One place is along river banks. When a river floods it drops the largest debris first directly next to the river creating a levee (see New Orleans). This levee is often made up of silt. Another place is farther out on the continental shelf, beyond the formation of the sandstone but before the shale in the open ocean. The grains are not visible like in shale but when rubbed against teeth it feels gritty, where as shale will feel smooth.

Picture from www.sfu.ca

• Shale - Forms in two different places:

  • The open ocean - due to settling of clay in the calm ocean water far from shore. The clay is carried there by rivers and streams draining the continents

  • Flood plains surrounding meandering rivers - During floods the water in rivers overflow their banks and settle along floodplains creating temporary lakes where the material being carried settles out. The finer the material the farther out is goes.

  • Forms flat layers that often break apart very easily.

• Mudstone - Contains both clay and silt causing it to have a resemblance to shale but does not form into layers, similar to siltstone.

   

  

  Picture from ocw.mit.edu

Chemical Sedimentary Rocks

Most are formed on or near standing water

• Chert - Forms in the open ocean from the deaths of animals with silicic (quartz) shells. Usually only occurs in the absence of animals with carbonate shells.

• Coal - Usually when plants die they degrade due to the action of bugs and microorganisms but in swamps there is so much plant life that there is not enough

• Limestone - Most aquatic animal life makes their shells out of a variety of carbonate, all of which react with acid. Making identifying a limestone rather simple. The type of limestone is a little more complex.

  • Fossiliferous Limestone - Formed usually on the continental shelf from the large shells of animals like clams and snails cemented together with microscopic calcium carbonate shells, called lime mud.

   

  

   

  • Chalk - Formed from the compaction of a microscopic animal called coccoliths. Similar to fossiliferous limestone except the rock is typically all white and contains only one type of fossil.

   

  

  Picture from csmres.jmu.edu

   

  • Micrite - This is just fossiliferous limestone without the fossils. A rock composed of mostly lime mud. It forms a little further out from the shoreline than fossiliferous limestone where the mud can float on the water but it is too deep for large animal life.

   

  

   

  • Crystalline Limestone - An inorganic type of limestone that usually forms in shallow lagoons or lakes. The water in the lagoon becomes saturated in calcite and then the water starts to evaporate causing calcite to precipitate out of the rock. This is common where water is periodically added to the lagoon or the lake after a lot of calcite is precipitated out.

   

  

  Picture from ocw.mit.edu

   

  • Oolitic Limestone - This is another inorganic type of limestone where limestone mud is deposited along the continental shelf edge and the naturally movement of the water rolls the mud around creating little balls of calcite.

   

  

  Picture from csmres.jmu.edu

   

  • Travertine - This is the type of limestone formed in caves from the deposit of calcite along stalactites, stalagmites and other flowstones. Since the structures are built up layer by layer, this causes the rocks to banded appearances.

• Rock Gypsum - Similar to crystalline limestone except the water is supersaturated with gypsum instead of calcite. Usually formed in lagoons or shallow lakes. Often the environment is going to be hot and dry as well.

• Rock Salt - Similar again to Rock Gypsum or Crystalline Limestone except the water is supersaturated with halite (aka salt). Usually formed in lagoons or shallow lakes also (think Great Salt Lake). Often the environment is going to be hot and dry as well.

Maturity

• When classifying clastic sedimentary rocks, it is often easiest to think of them in levels of maturity. What the maturity of a rock is is how far the sediment in the rock has traveled before becoming cemented together.

• Think of the mountain top of the picture as the beginning and the ocean basin as the end. The further sediment is towards the end, the more mature it will be.

• Immature - The sediment at the beginning. The grains usually contain large pebbles, sometimes even cobbles and boulders mixed together with fine sediment like silt sand and clay. The roundness of the grains can also vary from angular to rounded. Minerals include quartz, feldspars, micas, clays and rock fragments. Rock types include Conglomerate and Breccia usually.

• Submature - The sediment just past the beginning but usually still along the mountain side. The grains are smaller but still contain a variety of smaller sizes like large sand grains through clay. Grains are usually still angular through well rounded. Minerals include all the immature minerals like quartz, micas, clays and rock fragments. Rock types include Arkose and Lithic Sandstone.

• Mature - The sediment along the end of the system. The grains are often sorted completely in different sizes where sand is with sand, silt with silt and clay with clay. The minerals are often just quartz or clay and again grouped together. Rock types include Siltstone, Shale, and Quartz Arenite.

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